The present invention relates to an angular reference apparatus and particularly to a new and improved vibrating gyroscope.
As strapdown inertial navigation technology continues to progress, there is a need to develop low-cost angular rate sensors. These applications include the need for low cost, small size, lifetime calibration, no thermal control, high reliability, high bandwidth and rapid reaction.
Prior art U.S. Pat. Nos. 3,913,405 and 4,019,391 describe free gyroscopic sensors. One of the problems with these prior art instruments is their sensitivity for a combination of nonlinear spring rate, damping and tuning errors along different axes. This necessitates ellipticity control to dynamically tune the instrument as described in U.S. Pat. No. 4,079,630. However, a bias error remains, dependent on the direction of vibration in a complex way. The bias error is not stable and requires constant rotation (carrouselling) or elaborate computer modeling to cancel.
Prior U.S. Pat. No. 3,924,474 uses a viscous gas fill to provide damping in order make the instrument respond to the input angular rate. The viscous gas fill, however, reduces the Q-value of the instrument to a point where a large drive torque requirement causes significant errors.
FIG. 1 shows prior art U.S. Pat. No. 3,913,405 which uses orthogonally located forcers 7a, 7b (not shown 9a, 9b) and pickoffs 8a, 8b (not shown 10a, 10b) that excite and sensor angular vibrations of the proof masses about their GG:s. The proof masses 13a and 13b are connected by a rod 1. The nodal points of the vibrating proof masses are coincident with the proof masses' centers of gravity along axes 21--21 and 22--22. The rod features necked down sections 40 and 41 which are coincident with the nodal points, forming a nodal suspension system. The flow flexural stiffness of the necked down sections is required to produce a high Q-value. The low torsional stiffness about the input axis (Y-axis) of the necked down portion, however, results in a very low mechanical resonant frequency. FIG. 2 shows the rotational symmetry of the instrument and its orthogonally located forcers through section A--A in FIG. 1.
FIG. 3 shows prior art U.S. Pat. No. 4,019,391 which uses a rod with low flexural stiffness. Because the proof masses cannot be supported by the rod, flexures 42 and 43 are used as nodal suspensions. The flexures used afford a torsionally stiff suspension. Unfortunately, the flexures are hard to fabricate, hard to balance and lead to a low Q-value of the vibrating proof masses.
Accordingly, the need exists for an improved angular reference apparatus, fully described in the following.